/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.hadoop.hbase.master;

import edu.umd.cs.findbugs.annotations.Nullable;

import java.io.IOException;
import java.util.List;
import java.util.Map;

import org.apache.hadoop.conf.Configurable;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hbase.ClusterMetrics;
import org.apache.hadoop.hbase.HBaseIOException;
import org.apache.hadoop.hbase.ServerName;
import org.apache.hadoop.hbase.Stoppable;
import org.apache.hadoop.hbase.TableName;
import org.apache.hadoop.hbase.client.RegionInfo;
import org.apache.hadoop.hbase.conf.ConfigurationObserver;
import org.apache.yetus.audience.InterfaceAudience;

/**
 * Makes decisions about the placement and movement of Regions across RegionServers.
 *
 * <p>Cluster-wide load balancing will occur only when there are no regions in
 * transition and according to a fixed period of a time using {@link #balanceCluster(Map)}.
 *
 * <p>On cluster startup, bulk assignment can be used to determine
 * locations for all Regions in a cluster.
 *
 * <p>This class produces plans for the
 * {@link org.apache.hadoop.hbase.master.assignment.AssignmentManager}
 * to execute.
 */
@InterfaceAudience.Private
public interface LoadBalancer extends Configurable, Stoppable, ConfigurationObserver {
    /**
     * Master can carry regions as of hbase-2.0.0.
     * By default, it carries no tables.
     * TODO: Add any | system as flags to indicate what it can do.
     */
    String TABLES_ON_MASTER = "hbase.balancer.tablesOnMaster";

    /**
     * Master carries system tables.
     */
    String SYSTEM_TABLES_ON_MASTER = "hbase.balancer.tablesOnMaster.systemTablesOnly";

    // Used to signal to the caller that the region(s) cannot be assigned
    // We deliberately use 'localhost' so the operation will fail fast
    ServerName BOGUS_SERVER_NAME = ServerName.valueOf("localhost,1,1");

    /**
     * Set the current cluster status.  This allows a LoadBalancer to map host name to a server
     *
     * @param st
     */
    void setClusterMetrics(ClusterMetrics st);

    /**
     * Set the master service.
     *
     * @param masterServices
     */
    void setMasterServices(MasterServices masterServices);

    /**
     * Perform the major balance operation for cluster, will invoke {@link #balanceTable} to do
     * actual balance. Normally not need override this method, except SimpleLoadBalancer and
     * RSGroupBasedLoadBalancer.
     *
     * @param loadOfAllTable region load of servers for all table
     * @return a list of regions to be moved, including source and destination, or null if cluster is
     * already balanced
     */
    List<RegionPlan> balanceCluster(Map<TableName, Map<ServerName, List<RegionInfo>>> loadOfAllTable) throws IOException;

    /**
     * Perform the major balance operation for table, all class implement of {@link LoadBalancer}
     * should override this method
     *
     * @param tableName      the table to be balanced
     * @param loadOfOneTable region load of servers for the specific one table
     * @return List of plans
     */
    List<RegionPlan> balanceTable(TableName tableName, Map<ServerName, List<RegionInfo>> loadOfOneTable);

    /**
     * Perform a Round Robin assignment of regions.
     *
     * @param regions
     * @param servers
     * @return Map of servername to regioninfos
     */
    Map<ServerName, List<RegionInfo>> roundRobinAssignment(List<RegionInfo> regions, List<ServerName> servers) throws HBaseIOException;

    /**
     * Assign regions to the previously hosting region server
     *
     * @param regions
     * @param servers
     * @return List of plans
     */
    @Nullable
    Map<ServerName, List<RegionInfo>> retainAssignment(Map<RegionInfo, ServerName> regions, List<ServerName> servers) throws HBaseIOException;

    /**
     * Get a random region server from the list
     *
     * @param regionInfo Region for which this selection is being done.
     * @param servers
     * @return Servername
     */
    ServerName randomAssignment(RegionInfo regionInfo, List<ServerName> servers) throws HBaseIOException;

    /**
     * Initialize the load balancer. Must be called after setters.
     *
     * @throws HBaseIOException
     */
    void initialize() throws HBaseIOException;

    /**
     * Marks the region as online at balancer.
     *
     * @param regionInfo
     * @param sn
     */
    void regionOnline(RegionInfo regionInfo, ServerName sn);

    /**
     * Marks the region as offline at balancer.
     *
     * @param regionInfo
     */
    void regionOffline(RegionInfo regionInfo);

    /*
     * Notification that config has changed
     * @param conf
     */
    @Override
    void onConfigurationChange(Configuration conf);

    /**
     * If balancer needs to do initialization after Master has started up, lets do that here.
     */
    void postMasterStartupInitialize();

    /*Updates balancer status tag reported to JMX*/
    void updateBalancerStatus(boolean status);

    /**
     * @return true if Master carries regions
     */
    static boolean isTablesOnMaster(Configuration conf) {
        return conf.getBoolean(TABLES_ON_MASTER, false);
    }

    static boolean isSystemTablesOnlyOnMaster(Configuration conf) {
        return conf.getBoolean(SYSTEM_TABLES_ON_MASTER, false);
    }

    static boolean isMasterCanHostUserRegions(Configuration conf) {
        return isTablesOnMaster(conf) && !isSystemTablesOnlyOnMaster(conf);
    }
}
